(19) session 19 halo-alkanes, ethers & amines

3/28/2012

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Session 19

Organic Chemistry, UNAM School of Medicine1

HaloHaloHaloHalo----alkanes (Alkyl Halides), Ethers & alkanes (Alkyl Halides), Ethers & alkanes (Alkyl Halides), Ethers & alkanes (Alkyl Halides), Ethers &

AminesAminesAminesAmines

Dr L.H.A. Prins (Ph.D.)

Dept. of Pharmacy

UNAM

Learning Outcomes

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� By the end of this session, the student should be able to:

�Understand the importance of halo-alkanes, ethers &

amines

�Describe the structure of halo-alkanes, ethers & amines

�Name halo-alkanes, ethers & amines according to

IUPAC system

�Describe the physical properties of halo-alkanes, ethers &

amines

Organic Chemistry, UNAM School of Medicine

3/28/2012

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HaloHaloHaloHalo----alkanes alkanes alkanes alkanes

(Alkyl Halides)(Alkyl Halides)(Alkyl Halides)(Alkyl Halides)


Importance??Importance??Importance??Importance??

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� Halogen-substituted organic compounds – widespread

throughout nature� ± 5000 halo-alkanes have been found in algae & various other

marine organisms

� Halogen-containing compounds - vast array of industrial applications, including their use as solvents, inhaled anaesthetics in medicine, refrigerants & pesticides


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� In spite of all the useful applications, some fluoroalkanes (Freons®)

are detrimental to earth’s upper ozone layer & are banned from use in most parts of the world

� Chlorofluorocarbons (CFCs) - Cl atoms initiate a reaction chain that damages the ozone layer, which is needed to protect life on earth

from harmful UV rays


• Hydrochlorofluorocarbon (HCFC) &

hydrofluorocarbon (HFC) compounds

such as CF3CHCl2 & CF3CH2F - now

being used as CFC replacements

Structure of haloStructure of haloStructure of haloStructure of halo----alkanesalkanesalkanesalkanes

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� Halogen (X) might be bonded to:� an alkynyl group (C=C-X),

� a alkenyl group (C=C-X),

� an aromatic ring (Ar-X), or

� an alkyl group (C-X)

� Primarily concerned with alkyl halides (halo-alkanes) -compounds with a halogen atom bonded to a saturated, sp3

hybridised C atom


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� Halogens ↑ in size going down periodic table (F < Cl < Br <I ) -

lengths of corresponding C-X bonds ↑ accordingly

� Additionaly, C-X bond strengths ↓ going down periodic table

(C-F > C-Cl > C-Br > C-I)



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� In order for anything to react with the halo-alkanes, the C-X

bond has to be broken

� Because that gets easier as you go from fluoride (F) to iodide (I),

the compounds get more reactive in that order

Iodoalkanes - most reactive


Fluoroalkanes – least reactive

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� Halogens: More electronegative (EN) than C� C-X bond = polar covalent bond, with C atom bearing a slight +

charge (δ+) & halogen a slight −−−− charge (δ-)

� This polarity results in a substantial dipole moment for all the halomethanes (Table –slide 7)


Halo-alkane C: Tetrahedral geometry: 109.5°°°° bond angle


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� Classification of halo-alkanes:

� Primary (1o): X attached to 1o carbon - RCH2-X

� Secondary (2o): X attached to 2o carbon - R2CH-X

� Tertiary (3o): X attached to 3o carbon - R3C-X


# of alkyl groups (R) attached to the C to which the X is bonded determines whether a halo-alkane (alkyl halide) is primary, secondary or tertiary

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Nomenclature of haloNomenclature of haloNomenclature of haloNomenclature of halo----alkanesalkanesalkanesalkanes

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� Common names - name of alkyl group, followed by name of halogen - with the “ine” ending of halogen name replaced by “ide” (i.e., fluoride, chloride, bromide, iodide)

� Examples:

� Systematic names (IUPAC) - alkyl halides named as substituted

alkanes (halo-alkanes)

- prefix names for the halogens end

with “o” (i.e. fluoro, chloro, etc.)Organic Chemistry, UNAM School of Medicine

Nomenclature of haloNomenclature of haloNomenclature of haloNomenclature of halo----alkanesalkanesalkanesalkanes

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� Examples:


I

2-bromo-5-methylheptane 1-chloro-5,5-dimethylhexane(1-chloro-3-neopentylpropane)

1-ethyl-2-iodocyclopentane

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Physical properties of haloPhysical properties of haloPhysical properties of haloPhysical properties of halo----alkanes alkanes alkanes alkanes

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� The polar C–X bond means that halo-alkanes have a substantial

dipole moment

� halo-alkanes are poorly soluble in H2O, but are soluble in

organic solvents (i.e. C-based solvents)

� They have boiling points which are similar to alkanes of

comparable molecular weight


EthersEthersEthersEthers


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� Dimethyl ether (methoxymethane):An aerosol spray propellantDisadvantage of being flammable

� Diethyl ether (ethoxyethane):A common low boiling solvent (b.p. 34.6°C) & an early anaesthetic

� Anisole (methoxybenzene):A precursor to perfumes, insect pheromones & pharmaceuticals


Structure of ethersStructure of ethersStructure of ethersStructure of ethers

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� Ethers (R-O-R) can be considered derivatives of water (H-O-H) or alcohols (R-O-H) by replacing an H with an R group

� R groups in ethers can be: � alkyl (C-C-O-C-C),

� alkenyl (C=C-O-C=C), or

� aryl (aromatic ring) (Ar-O-Ar)

� If R groups are the same (R-O-R): Called symmetrical ethers

� If R groups are different (R-O-R’): Called unsymmetrical ethers

� Ethers can be cyclic or acyclic


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Nomenclature of ethersNomenclature of ethersNomenclature of ethersNomenclature of ethers

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� Common names - consist of the names of the two alkyl

substituents (in alphabetical order), followed by the word “ether”

The smallest ethers are usually named by their common names

� Examples:


ethyl methyl ether diethyl ether tert-butyl isobutyl ether


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� Systematic names (IUPAC) - named as alkoxy (RO−) substituted alkanes, alkenes & alkynes. The substituents are named by replacing the “yl” ending in the name of the alkyl substituent

with “oxy”

� Remember:

“Alkoxyalkane”, “Alkoxyalkene”, “Alkoxyalkyne”

� The group in the chain that has greatest # of C atoms is designated the parent compound

� In the case of aromatic ethers, the benzene ring is the parent compound


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� Examples:


2-methoxybutane 1-ethoxy-3-methyl-pentane

1,4-diisopropoxybutane

O

O

1,4-dimethoxybenzene(p-dimethoxybenzene)

Physical properties of ethers Physical properties of ethers Physical properties of ethers Physical properties of ethers

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� Ethers have nearly the same geometry as H2O � R-O-R bonds have an approximately tetrahedral bond angle (112°

in dimethyl ether) & O atom is sp3-hybridised

� The electronegative O atom gives ethers a slight dipole moment� Boiling points of ethers are often slightly higher than boiling points

of comparable alkanes (Van der Waals forces & dipole-dipole interactions must be overcome)


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� Boiling point comparison of Ethers & Hydrocarbons:



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� Volatility & Solubility:

� H atoms in H2O have ↑ partial + charge than H atoms on ether

� H2O: Charge is localised (only on) the H’s

� Ether: Charge is delocalised (spread throughout) the alkyl groups

� ∴ Charge is stronger in water than in ethers

� Because of delocalised nature of + charge on molecule's H atoms,

the H’s can not partake in H bonding


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� Volatility & Solubility:

� ∴ Ethers only form H bonds to other molecules that have H atoms with strong partial + charges

� ∴ Ether molecules can not form H bonds with other ether molecules → leads to high volatility of ethers

� Ethers → limited solubility in water (used for extractions during chemical synthesis)


AminesAminesAminesAmines


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� Many amines are biologically active:


=>

Structure of aminesStructure of aminesStructure of aminesStructure of amines

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� Nitrogen (N) atom in an amine is sp3 hybridised• The 3 groups & the unshared e- pair around N result in a tetrahedral

geometry (Molecular geometry)

• If unshared e- pair is not considered - shape of the amine is trigonalpyramidal

• Partial negative charge (δ-) = localised in region of lone pair e-


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� Amine classification:

� primary (1°) = 1 x C group bonded to N atom

� secondary (2°) = 2 x C groups bonded to N atom

� tertiary (3°) = 3 x C groups bonded to N atom

� quaternary (4°) = 4 x C groups bonded to N atom (N has + formal charge)



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� Further amine classification:

• Aliphatic amines: amines in which N is bonded only to alkyl groups

• Aromatic amines: amines in which N is bonded to one or more aryl

groups

• Heterocyclic amines: amines in which N is one of the atoms of a ring


NH2

Aminobenzene(Aniline)

N

H

N

N-propylpropan-1-amine(Dipropylamine)

Pyrrolidine Pyridine

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Nomenclature of aminesNomenclature of aminesNomenclature of aminesNomenclature of amines

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� Common names - names of the alkyl/aryl groups bonded to N, in alphabetical order, followed by “amine”

Entire name – written as one word! (Unlike common names of

alcohols, ethers & alkyl halides)

� Examples:



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� Systematic names (IUPAC)

- Name is based on longest C chain

- “e” of parent hydrocarbon replaced with “amine”

- # identifies the C to which the N is attached

- # can appear before name of parent hydrocarbon or before

“amine”

- name of alkyl group bonded to N - preceded by an“N” (in italics)

indicating that the group is bonded to a N rather than to a C


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� Examples:

- Substituents:

Listed in alphabetical order, regardless of whether they are

attached to the N or to parent hydrocarbon

A # or an “N” is assigned to each substituent

Chain - numbered in direction that gives functional group suffix

lowest possible #Organic Chemistry, UNAM School of Medicine


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� Examples:


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� Systematic names (IUPAC)…cont’d

- If NH2 group is classified as substituent in a compound – called an

amino group

� Examples:

-The common arylamines have the following names:


Physical properties of amines Physical properties of amines Physical properties of amines Physical properties of amines

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� Boiling points:

� 1° & 2° amines: Can form H-bonds with each other & H2O

� 3° amines: Can not form H-bonds with each other, but can form H-

bonds with H-bond donors such as H2O

� 3° amines: Have lower boiling points than 1° or 2° amines of

comparable molecular weights


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� H2O Solubility:

� 1°, 2° & 3° amines: Can all form H-bonds with H2O

� Low-molecular weight amines are generally H2O soluble



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� Basicity: Amines are weak organic bases � React with H2O to produce alkylammonium ions & hydroxide

anions

� React with acids to produce alkylammonium salts


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� Basicity: Amines are weak organic bases � Basic mechanism:


Donates an e- pair & accepts a proton (H+)

Thank you

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END


Health & Medicine

(19) session 19 halo-alkanes, ethers & amines